1. Field of the Invention
The present invention relates to a semiconductor device. Specifically, the present invention relates to a semiconductor device for exchanging (receiving and transmitting) data by wireless communication. Further specifically, the present invention relates to a configuration of a modulation circuit which modulates data exchanged in a wireless communication.
2. Description of the Related Art
In recent years, an individual identification technology using wireless communication (hereinafter referred to as a wireless communication system) has attracted attention. In particular, an individual identification technology in which a semiconductor device utilizing an RFID (radio frequency identification) technology (also referred to as an RFID tag, an RF tag, an IC (integrated circuit) tag, an IC chip, a wireless tag, or an electronic tag) is used as a data carrier which exchanges data by wireless communication has attracted attention. The semiconductor device which can perform data reception and transmission wirelessly has started to be used for production, management, or the like of an individual object and has been developed to be applied to personal authentication.
The wireless communication system herein refers to a communication system in which date is wirelessly exchanged between a transceiver with a power supply source such as a reader/writer, and a transceiver such as a semiconductor device.
The wireless communication system does not need a physical connection between the reader/writer and the semiconductor device. That is, as log as the semiconductor device exists in a region specified by the reader/writer, the reader/writer communicates with the semiconductor device and data can be exchanged with the semiconductor device.
In order to extend the communication distance between the reader/writer and the semiconductor device, research and development for increasing a power supply efficiency from the reader/writer to the semiconductor device have been actively performed (for example, see Patent Document 1).
Meanwhile, in the wireless communication system, when data in a plurality of semiconductor devices is read by a reader/writer at the same time, a distance between the reader/writer and each semiconductor device (hereinafter referred to as a communication distance) is not the same. Further, the communication distance may change from time to time, such as in a case where a carton of products each provided with a semiconductor device is passed in front of a reader/writer using a forklift truck. In general, electric power is attenuated in proportion to the squared distance between a radiant point of electric power and a measurement point of the electric power. That is, although electric power supplied from the reader/writer to the semiconductor device varies depending on the communication distance, the semiconductor device needs to operate stably under different electric power conditions.
Therefore, particularly when the communication distance is extremely short, such as in the case where the reader/writer and the semiconductor device are in contact with each other, a large amount of electric power is supplied to the semiconductor device, whereas a sufficient amount of electric power is not supplied in the case where the communication distance is long. When a large amount of electric power is supplied to the semiconductor device, the semiconductor device cannot correctly demodulate a signal from the reader/writer and malfunctions, so that an internal element of the semiconductor device is deteriorated. In the worst case, there is a concern that the semiconductor device itself is destroyed.
On the contrary, there is a method in which a protection circuit is provided inside a semiconductor device in order to suppress deterioration and destruction of an element, and voltage which is greater than or equal to a certain voltage value is prevented from being applied to an element of the semiconductor device even when a large amount of electric power is supplied to the semiconductor device, by provision of the protection circuit which divides the electric power (for example, see Patent Document 2 and Patent Document 3).